At present organophilic pervaporation (OPV) is a promising separation technique that involves the use of non-porous polymeric membranes, which are brought into contact with a liquid stream containing two or more miscible components of which one or more organic solvents. In contrast to hydrophilic pervaporation, which is for instance applied in solvent dewatering, OPV membranes preferentially remove the organic components from the mixture due to their higher affinity for, and/or quicker sorption in the hydrophobic membrane. The driving force for the transport of components through pervaporation membranes is the chemical potential gradient and more specifically the partial vapour pressure gradient of the components at the feed and permeate side. The mass transport in pervaporation is generally described by the solution-diffusion model, which is based on a three-step transport mechanism consisting of (i) sorption of the permeant from the feed mixture at the upstream side of the membrane, (ii) diffusion of the permeant through the membrane, and (iii) desorption of the permeant at the downstream side of the membrane. The vaporous permeate is subsequently condensed to obtain a liquid product. According to the solution-diffusion mechanism, the pervaporation flux is a function of the solubility in and diffusivity through the membrane. Membrane selectivity is thus affected by the solubility of a compound in the polymer, which is determined by the permeate-membrane interaction, and the diffusivity which is generally governed by the molecular size, shape and mass of the permeant.
The number of commercial OPV membranes that combine a high selectivity with an elevated pervaporation flux, and moreover show long-term stability in organic solvents is however restricted at present. OPV did therefore not yet realize a breakthrough in industrial processes so far, despite its clear environmental and economical advantages and the great application potential in the process industry. Several polymers have been used for the synthesis of OPV membranes, e.g. polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), ethylene-propylene-diene terpolymer (EPDM), polyurethaneurea (PU), poly(ether-block-amide) (PEBA) and poly(1-trimethylsilyl-1-propyne) (PTMSP).
U.S. Pat. No. 6,316,684 provides separation membranes comprising a polymer with particles dispersed therein. In the examples, poly(4-methyl-2-pentyne) (PMP) and poly(1-trimethylsilyl-1-propyne) (PTMSP) membranes are cast on a glass plate.
PTMSP is a substituted polyacetylene that combines a rigid backbone chain with trimethylsilyl side-groups. These bulky groups restrict rotational mobility and limit the polymer's ability to pack together. PTMSP is a hydrophobic glassy polymer (Tg>300° C.) with an extremely high free volume fraction (up to 25%) and it exhibits intrinsic nanoporosity. PTMSP is one of the most studied polymers for gas separation applications. PTMSP-based gas separation membranes have already been disclosed in De Sitter et al. (in “Silica filled poly(1-trimethylsilyl-1-propyne) nanocomposite membranes: relation between the transport of gases and structural characteristics”, Journal of Membrane Science vol. 278 (2006), pp. 83-91) wherein a method for preparing a filled polymeric membrane is described. Nonporous PTMSP membranes have also been applied in the pervaporative separation of aqueous alcohol mixtures, and recently also in nanofiltration of alcoholic feed solutions. However, these dense PTMSP membranes generally exhibited low permeate fluxes. WO 2009/027376 provides a PTMSP layer with a thickness of about 30 μm cast upon a porous polyacrylonitrile substrate.
Therefore, at present there is a pressing need for OPV membranes that combine a high alcohol/water selectivity with an elevated pervaporation flux, and moreover show long-term stability in strongly swelling organic solvents.
The present invention aims at providing OPV membranes that combine the required characteristics: a high selectivity, an elevated pervaporation flux and a long-term stability. The present invention also aims to provide methods for manufacturing such membranes. The present invention also aims to provide membrane separation processes having an improved performance over processes of the prior art. Particularly, the present invention aims to provide an improved pervaporation process, in particular for separating alcohols from dilute aqueous mixtures.